Alkaline secondary batteries typified by a nickel-hydrogen battery and nonaqueous electrolyte secondary batteries typified by a lithium ion battery are widely used as power supplies for driving portable electronic equipment such as cell phones including smartphones, portable computers, PDAs, and portable music players. In addition, alkaline secondary batteries and the nonaqueous electrolyte secondary batteries are also widely used for power supplies for driving electric vehicles (EVs) and hybrid electric vehicles (HEVs, PHEVs) and in stationary storage battery systems for suppressing the variation in output power of photovoltaic generation, wind power generation, and the like, and for peak shifts in system power in order to store electric power during the night time and to use the electric power during daytime.
In particular, the batteries for EVs, HEVs, and PHEVs and for the stationary storage battery system are required to have high capacity and high output characteristics, and hence each battery is upsized and a number of batteries are connected in series or parallel when used. To address this, in these applications, prismatic secondary batteries are generally used from the viewpoint of space efficiency. A prismatic secondary battery that further needs physical strength commonly employs, as an outer body of the battery, a metal prismatic outer body having a mouth and a metal sealing plate for sealing up the mouth.
Such a prismatic secondary battery, for example, a prismatic nonaqueous electrolyte secondary battery, is produced as follows. For example, both faces of a positive electrode substrate made from, for example, a long sheet of aluminum foil, are coated with a positive electrode active material mixture containing a positive electrode active material to prepare a positive electrode sheet. Separately, both faces of a negative electrode substrate made from, for example, a long sheet of copper foil, are coated with a negative electrode active material mixture containing a negative electrode active material to prepare a negative electrode sheet.
Next, the positive electrode sheet and the negative electrode sheet are stacked interposing a separator made from, for example, a microporous polyethylene film therebetween, and the positive electrode sheet and the negative electrode sheet are spirally wound on a cylindrical winding core while insulating the positive electrode sheet and the negative electrode sheet from each other through the separator to prepare a cylindrical wound electrode assembly. Then, the cylindrical wound electrode assembly is pressed with a pressing machine to form a flat wound electrode assembly. Next, a positive electrode collector electrically connected to the positive electrode sheet is electrically connected to a positive electrode terminal that is insulated from a sealing plate, while a negative electrode collector electrically connected to the negative electrode sheet is electrically connected to a negative electrode terminal that is insulated from a sealing plate. Then, the flat wound electrode assembly is wrapped with a member having insulating characteristics and stored in a metal prismatic outer body; a mouth portion of the prismatic outer body is sealed with a sealing plate; an electrolyte is poured from a electrolyte pour hole provided on the sealing plate; and finally the electrolyte pour hole is sealed to produce the prismatic nonaqueous electrolyte secondary battery.
Such a prismatic secondary battery required to have high capacity and high output characteristics is required to have much higher safety than that of secondary batteries for portable small equipment. Especially, in the case of a nonaqueous electrolyte secondary battery that uses a material having very high reactivity, for example, as shown in US Patent Publication No. 2010/0233529 (U52010/0233529 (A1)) and U.S. Pat. No. 7,781,088 specification (U.S. Pat. No. 7,781,088 (B2)), this nonaqueous electrolyte secondary battery is equipped with a gas release valve for releasing internal pressure when the pressure in a battery outer body is increased and a current interruption mechanism for interrupting electrical connection between an external terminal and an electrode assembly in the outer body.
The metal sealing plate used for the prismatic secondary battery includes at least a mouth for attaching a positive electrode terminal, a mouth for attaching a negative electrode terminal, a gas release valve, and an electrolyte pour hole. The metal sealing plate commonly has a rectangular shape, a chamfered rectangular shape, a rounded rectangular shape, or an oval shape. The mouth for attaching a positive electrode terminal and the mouth for attaching a negative electrode terminal are arranged on both end sides in a longitudinal direction of the sealing plate, and each of the gas release valve and the electrolyte pour hole is provided between the negative electrode terminal and the positive electrode terminal on the sealing plate.
Meanwhile, the prismatic secondary battery is mass-produced and thus is preferred to have a sealing plate with any identification code for providing traceability during an assembly process and after the assembly. Such an identification code can be easily formed by printing, laser marking, or seal-affixing on the sealing plate. However, when an electrolyte is poured from the electrolyte pour hole into the prismatic outer body, there is a possibility that the electrolyte disperses and adheres to the identification code formed on the sealing plate. Adherence of the electrolyte to the identification code deteriorates readability of the identification code. There is also a possibility that the traceability is lost due to corrosion or damage of the identification code by electrolyte adhered to the identification code or by acid generated by a reaction of electrolyte and moisture in the air.